The determination of charged energy states of semiconductor and insulator materials, especially of oxide layers on semiconductors, forms a basic task of semiconductor research and production, since the energy states (the deep levels) substantially determine the properties of semiconductor devices. The first widely accepted method for the quick examination of deep levels was proposed by D. V. Lang (J. Appl. Phys. 45, pp. 3023-3032, 1974) and he called his method deep level transient spectroscopy or in short as DLTS method. This method was used in its first implementation for detecting transient capacitances, and later B. W. Wessels also proposed (J. Appl. Phys. 47, pp. 1131-1133, 1976) the DLTS measurement of current transients. The principial identity of these two methods was disclosed by J. A. Borsuk and R. M. Swansen (IEEE Trans. Electron. Dev. ED-27, pp. 2217-2225, 1980), and it was stated that there is no inherent advantage in the use of either current or capacitance transients.
In the original Lang method, a double boxcar integrator was used for averaging the transient signals. For improvement of this method, L. C. Kimerling (IEEE Trans. Nucl. Sci. NS-23, 1976) first proposed the usage of a lock-in amplifier for signal averaging. In principle, the lock-in amplifier can provide a better signal to noise ratio, however, in the particular application suggested by that paper, its accuracy and reliability has not yet been satisfactory. The analysis of the problems was disclosed by D. S. Day et al. (J. Appl. Phys. 50, pp. 5093-5096, 1979).
For increasing the accuracy, a special timing and the usage of a wideband synchronous detector (lock-in amplifier) has been proposed by the present applicant in U.S. Pat. No. 4,437,060.
In order that the measurements carried out with different repetition frequencies be comparable with each other, in this Patent, a phase adjustment was used which is independent of the repetition frequency of the exciting pulses. The passage of the measuring signal was blocked during the existence of the exciting pulses and of a dead period corresponding to a frequency-independent phase-angle, and the squarewave of the lock-in amplifier was synchronized to the terminating moments of the dead periods. The length of the dead period was adjusted depending on the time required for the decaying of the switching transients of the measuring unit. According to the considerations disclosed in that publication, the length of the dead period had to be about fifteen times greater that the response time of the measuring unit (detector), because in such a period the transients would decay to 10.sup.-5 times the original value. The required accuracy could be reached only by such a long waiting time.
If it is taken into account that the duration of switching transients (the response time of the detector) is typically 5 .mu.s, then the minimum value for the dead period was 75 .mu.s. For obtaining the required accuracy, the combined duration of an exciting pulse and of the dead period can not be longer than 10% of the repetition period of the exciting pulses, therefore, the repetition period of the exciting pulses can not be shorter than 750 .mu.s even if indefinitely short exciting pulses are chosen. With practical pulse-lengths, the minimum repetition period time was about 1 ms. Due to practical considerations, the period time of the exciting pulses can not be longer than about 1 s, and with these limit values, the possible range of adjustment of the period time of the exciting pulses will not be greater than three decimal orders of magnitude. This range of adjustment is required for determining the temperature dependence of activation energy. For widening the scale of measurements, the increase of this range of adjustment has an outstanding significance.
A different method has been proposed by M. Schulz and H. Lefevre in German Pat. No. 2.631.783 (Appl. Phys. 12, pp. 45-53, 1977) which can be regarded as a special improvement of the averaging by a boxcar amplifier and this is called the double correlation DLTS method or in short as the DDLTS method. The essence of the double correlation DLTS method of Schulz et. al. lies in the application of two exciting pulses with different shapes, and, following these exciting pulses, respective samples are taken in two different moments from the transient capacitance signals by using the well known Lang's method. The sign of the sampling pulses are different and the sign-order is inverted following the respective exciting pulses.
In contrast to the original Lang method, the DDLTS technique is less sensitive to DC offset drifts and it facilitates the determination of the profile of the spatial distribution of deep levels. The comparison of various correlation methods has been disclosed by C. R. Crowell et. al. in Solid State Electronics Vol. 24. pp. 25-36, 1981. It should be noted that for the implementation of the DDLTS method, an apparatus is required which is different from those used for the DLTS technique.
In the DDLTS method as proposed by Schulz et. al., the time windows used for the sampling pulses take only a small fraction of the time elapsed between subsequent exciting pulses, and from this it follows that the signal to noise ratio of this method is less preferable. For performing double correlation, specially designed multi-channel analog processing circuits are required. The application of a large number of analog circuits decreases the signal to noise ratio and, in this way, the sensitivity. With such an apparatus, capacitance transients smaller than 10.sup.-3 pF can not be detected.
The object of the invention is to provide a method by which an increased signal to noise ratio and a higher sensitivity can be reached during the usage of the DLTS technique and which enables the performance of all examinations that have been limited to the DDLTS technique so far.
A further object of the invention lies in the provision of a comparatively simple apparatus for carrying out the method.